Critical infrastructure—assets, systems, and networks, whether physical or virtual, that are so vital to the functioning of a society that their incapacitation would have a debilitating effect on security, national economic security, national public health, or safety—such as power grid systems, transportation systems, banking operations, and communication systems rely on highly accurate and precise timing that is synchronized to Coordinated Universal Time (UTC). For example, power grids rely on synchronized timing to enable various distributed devices, such as protection relays, phasor measurement units, merging units, event recorders, fault detectors, and control consoles, to stay synchronized to each other and UTC with plus or minus 1 microsecond accuracy. As another example, communications systems, which strive for faster data rates, must achieve a low Bit Error Rate (BER), which requires highly accurate network synchronization. The frequencies at each node in the communication system have to be maintained more closely, more accurately, and with less drift in order to push data rates higher. Wireless broadband LTE-A communication systems require plus or minus 1.5 microsecond accuracy to UTC. Banking communications networks must maintain timing synchronization of data encryption and decryption equipment and for high speed transactions.
Global Navigation Satellite Systems (GNSS) such as the U.S. NAVSTAR Global Positioning System (GPS), the European Galileo positioning system, and the Russian GLONASS system are increasingly relied upon to provide synchronized timing that is both accurate and reliable. (Reference is made to GPS below, by way of example and simplicity, but similar characteristics and principles of operation apply to other GNSS.) GPS provides highly accurate and precise timing around the world, and the equipment needed to receive and use GPS signals is widely available and cheap, allowing for timing signals to be received at many diverse locations. As shown in FIG. 1B, critical infrastructure such as power grid systems, transportation systems, banking operations, and communications systems all depend on GPS time reference. GPS is a constellation of twenty-four satellites, eighteen active, and six ready spares that orbit the earth in polar, equatorial, and diagonal orbits. The twenty-four satellites that make up the GPS network provide space-based positioning, navigation, and timing, which includes the distribution of precise time and precise time of day. As illustrated in FIG. 1A, GPS receivers simultaneously receive GPS signals from a number of satellites in the constellation. These receivers use the information contained in the signals to provide network synchronization, location, and navigation. The GPS network also includes earth based performance monitoring stations that constantly measure the time signals from each satellite as each satellite passes over a controlled site. These monitoring stations then send clock corrections back to an individual satellite if and when necessary.
A GPS signal is a direct sequence spread spectrum signal. The signal available for commercial use is that associated with Standard Positioning Service and utilizes a direct sequence bi-phase spreading signal with a 1.023 MHz spread rate placed upon a carrier at 1575.42 MHz (L1 frequency). Each GPS satellite transmits a unique pseudo-random noise (PN) code which identifies the particular satellite, and allows signals simultaneously transmitted from several satellites to be simultaneously received by a receiver, with little interference from one another. The pseudo-random noise code sequence length is 1023 chips, corresponding to 1 millisecond time period. One cycle of 1023 chips is called a PN frame. Thus, each received GPS signal in C/A (coarse acquisition) mode is constructed from the high rate 1.023 MHz repetitive PN pattern of 1023 chips. At very low received signal levels, the pseudo random pattern may be tracked, or otherwise used, to provide ambiguous system timing by processing many PN frames (e.g., 1000 repetitions over 1 second). A GPS receiver knows the PN codes of satellites in the constellation and may lock into a given GPS satellite by generating and shifting its code until the generated code lines up with the received code. The amount of shift along with knowledge of the distance to a GPS satellite enables the receiver to determine timing synchronized to the GPS satellite. In this process, the GPS receiver essentially measures the start times of PN frames for a multiplicity of received GPS signals.
Superimposed on the 1.023 MHz PN code is a low rate signal. This 50 Hz signal is a binary phase shift keyed (BPSK) data stream with bit boundaries aligned with the beginning of a PN frame. There are exactly 20 PN frames per data bit period (20 milliseconds). The 50 Hz signal modulates a Navigation Message which consists of data bits describing the GPS satellite locations, clock corrections, time-of-week information, and other system parameters. The absolute time associated with the satellite transmissions are determined in conventional GPS receivers by reading data contained within the Navigation Message of the GPS signal. In the standard method of time determination, a conventional GPS receiver decodes and synchronizes the 50 baud data bit stream (the 50 Hz BPSK data stream). The 50 baud signal is arranged into 30-bit words grouped into subframes of 10 words, with a length of 300 bits and a duration of six seconds. Five subframes comprise a frame of 1500 bits and a duration of 30 seconds, and 25 frames comprises a superframe with a duration of 12.5 minutes. The data bit subframes which occur every six seconds contain bits that provide the Time of Week to six second resolution. The 50 baud data stream is aligned with the C/A code transitions so that the arrival time of a data bit edge (on a 20 millisecond interval) resolves the absolute transmission time to the nearest 20 milliseconds. Precision synchronization to bit boundaries can resolve the absolute transmission time to less than a millisecond.
GPS time is steered to within one microsecond of UTC, with the exception of an integer number of leap seconds, which are added to UTC time but not to GPS time. The Navigation Message includes the offset between the GPS time and UTC time to a precision of 90 nanoseconds. Time intervals can be produced by a receiver using the 1 millisecond repetition rate of the PN code. Each millisecond of GPS satellite time can be time tagged by extracting the week number and time of week count from the standard GPS navigation message. Reference frequencies may be established by a GPS receiver by disciplining an oscillator using integrated code-phase measurements or by directly measuring the GPS satellite carrier frequency.
Using the above methods, GPS receivers can provide the highly accurate and precise timing required by critical infrastructure. However, dependence on GPS may be problematic for critical infrastructure. Dependence creates a single point of failure in which the loss of GPS signals could result in a serious operational disruption. GPS signals are vulnerable to a range of environmental and intentional disruptions. For example, geomagnetic storms may disrupt or distort GPS signals, which are relatively weak and, therefore, subject to geomagnetic radiation. Furthermore, GPS signals can be intentionally jammed or spoofed. GPS jammers, which generally transmit narrowband Gaussian noise signals near the L1 frequency, are cheap and widely available and can prevent receivers from acquiring GPS signals. GPS spoofers may broadcast counterfeit GPS signals containing misinformation causing receivers to generate incorrect timing information. Many conventional systems relying on GPS include atomic clocks that may be used to “holdover” while a GPS signal is down. However, reliance on these clocks for extended periods can cause increasingly inaccurate timing.
Accordingly, there is a need for a robust and resilient timing source for critical infrastructure that does not rely on GPS but still provides accurate synchronized timing to a wide geographic area.